Blog post
O-ring failure modes: how to read a failed seal
A failed O-ring usually shows its cause on its surface. A chewed edge on the low-pressure side is extrusion, a flat-sided oval is compression set, a corkscrew cut is spiral failure, and blisters are gas decompression. Section 10 of the Parker O-Ring Handbook (ORD 5700) groups the modes by appearance; this post puts that reading in the order a technician meets it at the bench.
Failure modes by appearance (Parker ORD 5700, section 10)
| Mode | What the ring looks like | Usual cause | Corrective direction |
|---|---|---|---|
| Extrusion and nibbling | Chewed, chipped or frayed edge on the low-pressure side | Too much clearance for the pressure, pressure spikes, a soft compound, swell, sharp gland edges | Close the clearance, a harder compound, a backup ring, check the fluid for swell |
| Compression set | Flat sides; the round section has become an oval and the ring no longer springs back | Heat, a compound with poor set resistance, too much squeeze, under-cure | A low-set compound, lower temperature at the seal, the correct gland depth |
| Abrasion | Flat wear on one side of a moving seal, sometimes with a shiny band | A finish too rough, or too smooth to hold a lubricant film; dirty fluid | Correct the surface finish, filter the fluid, a harder or lubricated compound |
| Spiral failure | Cuts that corkscrew around the ring at an angle to the section | The ring rolls and slides at once: long slow strokes, eccentricity, poor lubrication | Finish and lubrication, a harder ring, a backup ring, or a lobed section that resists rolling |
| Rapid gas decompression | Blisters, pits and internal fissures, often with no surface cut | Gas absorbed under pressure expands when the pressure drops quickly | Slower decompression, a compound rated for it, a harder or smaller section |
| Chemical swell or shrinkage | Soft, gummy and enlarged; or hard, shrunken and cracked | A compound that does not resist the fluid, or a fluid that changed | Match the compound to every fluid the seal sees, including cleaning agents |
| Heat hardening and cracking | Hard, brittle, with radial cracks on flexing | Service above the compound's band, or long aging near its limit | A compound rated for the temperature, or reduce the heat reaching the seal |
| Installation damage | Cuts, notches and skived slivers, usually one per ring | No chamfer, dry assembly over threads or edges, a wrong size stretched into place | Chamfers on the lead-in, break the edges, cover threads, lubricate, confirm the size |
| Weathering and ozone cracking | Fine cracks across the direction of stretch on the exposed surface | An ozone-sensitive compound, often nitrile, stored or used stretched in air or sunlight | An ozone-resistant family for exposed service, correct storage before use |
| Over-compression and over-fill | Pinched or split ring, sometimes with extruded flash at both edges | Too much squeeze or a gland too small for the swollen or thermally expanded ring | Correct the gland volume so the ring has room to swell and expand |
Separate the compound from the gland
Half the rows above are compound problems and half are gland problems, and the first question at the bench is which side the failure sits on. If the ring changed as material, by swelling, hardening, softening or cracking on its own, the compound did not match the fluid or the temperature. If the ring was cut, worn or squeezed out of shape while its material stayed sound, the gland, the pressure or the assembly did the damage.
The compound side is settled against the chemical compatibility guide and the temperature guide. The gland side is settled against the groove design guide and the squeeze guide, which give the dimensions a ring in each series needs. A flat gasket that swelled or hardened is read the same way, against the gasket material post.
Extrusion and the backup ring
Extrusion is the failure most often solved by adding a part rather than changing one. The ring is pushed into the gap between the two metal parts on the low-pressure side, and each pressure cycle nibbles a little more away. The three levers are the size of that gap, the hardness of the compound and the pressure; when the first two are fixed by the design, a backup ring closes the gap for the seal.
One backup ring goes on the low-pressure side when pressure acts in one direction, and two are used when it alternates. The backup ring page covers the styles, and the durometer guide explains why a harder compound resists extrusion but seals less well at low pressure.
Spiral failure and the lobed section
Spiral failure belongs to long-stroke reciprocating hydraulics. Part of the ring slides while another part rolls, the section twists, and the twist cuts the ring at an angle around its circumference. Surface finish, lubrication and a harder compound are the first remedies; where they are not enough, a section that cannot roll is the answer, either a backup ring behind the O-ring or a four-lobed ring in its place. The X-ring post explains when that swap makes sense.
What to record before you send it back
| Item | Why it matters |
|---|---|
| The part number or dash number, and the measured ID and cross-section | Confirms the ring was the size the print called for; the measuring post covers a used ring |
| The compound and hardness as installed | Half the modes are compound choices |
| Hours or cycles in service | Separates early failure from wear-out |
| Every fluid the seal saw, including cleaning agents | Swell and hardening come from the fluid list, not the main fluid alone |
| Temperature at the seal, continuous and peak | Heat drives compression set and hardening |
| Pressure, and whether it cycles or spikes | Extrusion and decompression are pressure failures |
| The motion: static, reciprocating or rotary | Abrasion and spiral failure need motion |
| Storage before installation and the cure date | Ozone cracking can start on the shelf; the shelf-life guide covers the storage practice |
| A photo of the ring in place before removal, and one after | The position of the damage is the evidence |
Reordering after a failure
Conquest supplies the replacement to the dash number and the compound the print names, with the material certification for the lot; when the reading points to a compound change, the drawing owner revises the print first and the new callout is what gets quoted; the FKM naming post covers what a brand name on that callout means. A ring that has taken a set without any other damage is the most common case, and the compression set guide covers what set is and how the gland is designed to survive it.
Sources
- Parker O-Ring Handbook ORD 5700 (2018), section 10 (failure analysis) and section 3 (extrusion, squeeze and stretch), read 2026-09-14
- SAE AS568 (title only)
Frequently asked questions
Why did my O-ring go flat?
Compression set: the ring was held squeezed at a temperature its compound could not recover from. Heat, over-squeeze and a compound with poor set resistance are the usual causes.
Why is the O-ring cut in a spiral?
It rolled and slid at the same time in a reciprocating gland. Long slow strokes, eccentricity and poor lubrication bring it on; a harder ring, a backup ring or a lobed X-ring section are the remedies.
Why did the O-ring swell?
The compound absorbed the fluid. Check every fluid the seal sees against the compatibility guide, including cleaning agents and additives, not only the main fluid.
Can an O-ring fail with no pressure on it?
Yes. Compression set, heat hardening, chemical attack and ozone cracking all happen at rest, and ozone cracking can start in storage if the ring is kept stretched or in light.
Should the material change after one failure?
Only if the reading is a compound failure. A ring cut on installation or nibbled by extrusion fails again in a better compound; the gland or the assembly is the fix.
Is a hard, cracked ring a heat problem or an age problem?
Usually heat, if the ring was in service; aging alone takes years within the storage bands. Radial cracks on flexing point to heat, fine surface cracks across the stretch direction point to ozone.
Related guides
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